Polyurethane self-priming topcoats

ABSTRACT

A corrosion-resistant coating which can be applied directly to a surface as self-priming topcoat comprising a polyurethane resin binder and a combination of pigments consisting essentially of an alkaline earth metal metaborate, zinc salts of benzoic acids, and an alkaline earth metal phosphate such as zinc-barium phosphate. In addition, the coating contains from 5-40 parts by weight of a titanium dioxide pigment, up to about 3.0 parts by weight of an oil soluble dispersant and up to about 50 parts by weight of at least one organic solvent.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

CONTINUATION APPLICATIONS

This application is a Continuation-In-Part of copending application Ser.No. 07/936,983 filed Aug. 25, 1992 which in turn is aContinuation-In-Part of copending application Ser. No. 07/812,174 filedDec. 20, 1991.

BACKGROUND OF THE INVENTION

This invention relates to novel coating compositions and morespecifically to corrosion-resisting coatings which can be applieddirectly to various surfaces particularly metal either as a high or lowgloss, self-priming topcoat.

Various surfaces and particularly metal surfaces require the protectionof coatings especially when the surfaces are exposed to a corrosiveenvironment. Metal surfaces of aircraft, for example, are exposed toseawater which require protection from corrosion. Specifically,aircraft, e.g., Navy aircraft, are exposed to seawater spray in additionto various acid-forming gases such as sulfur dioxide, carbon dioxide,etc. Moreover, in addition to aircraft, various machinery and equipmentin the industrial environments, where fossil fuels are used needprotection against corrosion. It is important therefore that the coatingbe resistant to corrosion, various chemicals, the weather and at thesame time be flexible and have good adhesion to the substrates.

Presently, coating systems comprise one or more films, i.e., anundercoat, a sealant and a topcoat. Aircraft, for example, traditionallyhave been coated with high performance two-component protective filmsconsisting of an epoxy primer and a polyurethane topcoat. The type ofepoxy primers used on the aircraft are designed to adhere to the metalsurface and help to improve the adhesion of the topcoat and togetherprevent corrosion of the metal. However, these undercoats require atopcoat, since the undercoats lack flexibility especially at lowtemperatures (-60° F.) resulting in extensive cracking particularly inhighly flexed areas of the aircraft. Moreover, the undercoats usuallylack weather resistance and generally cannot be formulated in differentcolors required for aircraft.

In comparison, the polyurethane coatings of this invention, provides thenecessary corrosion resistance, the required degree of flexibility, thedesired optical properties, and a resistance to weather and variouschemicals. To obtain these characteristics, the multi-film coatings usedheretofore generally required a dry-film thickness ranging up to about0.005 inches, e.g., up to about 10 mils or more which added considerableweight to the aircraft. In addition, the multi coats are time consumingto apply particularly since there is a drying time between eachapplication. Further, the removal of a two-coat system can be difficultand time consuming and generates high levels of volatile organic (VOC)emissions during the operations.

In accordance with this invention the corrosion-resistant coatingcomprise a polyurethane binder derived from the reaction of at least onepolyol, e.g., polyester polyol and an isocyanate, e.g., hexamethylenediisocyanate (HDI) in combination with a unique mixture ofcorrosion-inhibiting pigments consisting essentially of (1) an alkalineearth metal phosphate, e.g., zinc-barium phosphate, (2) zinc salts ofbenzoic acid or substituted benzoic acids, and (3) an alkaline earthmetal metaborate. All three of these metal compounds or salts areessential, in the stated relative proportions, to provide a singlehigh-gloss coating with the necessary corrosion resistance and adhesioncharacteristics required of a good top coat. In addition, titaniumdioxide (TiO₂) including spherical TiO₂ particles, e.g., vesiculatedbeads of TiO₂ are included as a pigment together with these threealkaline earth metal or zinc salts. The coating compositions of thisinvention may be applied, as one coat, directly onto various hardsurfaces such as metal and/or organic matrix composites, etc., and donot require an undercoat to provide a corrosion-resistant finish withdesired optical and adhesion properties.

SUMMARY OF THE INVENTION

A corrosion-resistant coating which can be applied directly to a surfaceas a self-priming topcoat comprising from about 25 to 55 parts by weightand preferably 30-50 parts of a polymeric urethane resin binder and acombination of three corrosion-resistant pigments consisting essentiallyof an alkaline earth metal metaborate (BUTROL-22), zinc salts of benzoicacids, and alkaline earth metal and/or zinc phosphates such as a complexzinc-barium phosphate, together with nonionic or anionic dispersingagents, TiO₂ pigments, and organic solvents.

Accordingly, it is an object of this invention to provide acorrosion-resistant coating which can be applied directly to a surfacee.g., metal, as a single coat.

It is another object of this invention to provide a coating which isflexible, resistant to corrosion, chemicals, and weathering, and hasgood adhesion characteristics.

It is still a further object of this invention to provide a coating foruse on military or civilian aircraft of reduced thickness to lower theweight thereon while at the same time providing the necessary corrosionresistance.

These and other objects of the invention are accomplished, in accordancewith this invention, by providing a corrosion-resistant coating capableof being applied as a single coating with appropriate opticalproperties.

THE PREFERRED EMBODIMENTS

This invention is directed to a corrosion-resistant coating whichfunctions as a primer and a topcoat. More specifically, this inventionrelates to a corrosion-resistant coating which comprises a major amount,e.g., 25 to 55 parts by weight of a urethane resin, i.e., polyurethaneresin binder, and minor amounts of corrosion-inhibiting pigments, i.e.,alkaline earth metal compounds and/or zinc salts. The unique combinationof corrosion-inhibiting pigments consist essentially of from about 1 to30 parts and preferably 5 to 25 parts by weight of an alkaline earthmetal phosphate, e.g., zinc or zinc-barium phosphate, 0.1 to 5 parts andpreferably 0.5 to 3 parts by weight of a zinc salt of a benzoic acid orsubstituted benzoic acid, and about 5.0 to 35 parts and preferably 10 to30 parts by weight of an alkaline earth metal metaborate. In addition tothe above metal compounds, depending on the opacity, etc., required ofthe coating, from 5.0 to 40 parts, and preferably from 10 to 35 parts byweight of titanium dioxide pigment, based on the total weight of thecoating, is added as an additional pigment. Up to about 100% of thetotal amount of TiO₂ may be in the form of vesiculated beads, e.g., from0 to 50% of the TiO₂ in the coating are beads. Generally, the coating isapplied as a high solids organic solution and therefore generallycontains from 0 to 3.0 and preferably about 0.1 to 1.0 parts by weightof at least one oil soluble nonionic or anionic dispersing or wettingagent such as Anti-Terra-204 or BYK-320. Anti-Terra-204 is a carboxylicacid of polyamine-amides and BYK-320 is a polyether modifiedmethylalkylpolysiloxane copolymers. Various other nonionic or anionicdispersing agents known as oil soluble dispersants or paint additivesmay be used in the coating. In addition, the coating contains from about0 to 50 parts, e.g. as needed or from 15 to 35 parts by weight of thetotal coating of at least one organic solvent, e.g., Mil-T-81772 orvarious mixtures of paint solvents.

The organic binder of the coating comprises a polyurethane, and moreparticulary an aliphatic polyurethane derived from the reaction of apolyol and a multi-functional aliphatic isocyanate. The polyol, e.g.polyester polyol, is preferably used as a solution in an organic solventsuch as toluene, xylene, n-butyl acetate, propylene glycol monomethylether acetate, methyl ethyl ketone, etc. The polyisocyanate can be usedas a 100% solid, but can be diluted also with any of the above organicsolvents. The hydroxyl number of the polyols, i.e., polyester polyolsand the isocyanate (NCO) content or the equivalent weights of thepolyisocyanate and polyol are determined in order to obtain the desiredpolyurethane. The preferred polyols and polyisocyanates are reacted inapproximately stoichiometric amounts so that the NCO to OH ratio rangesfrom about 0.85 to 1.4 equivalent of the NCO to 1.0 equivalent of theOH, or at about a 1.0 to 1.0 ratio of the NCO to OH.

The combination of metal salts and/or pigments is unique and consistsessentially of specific amounts of an alkaline earth metal phosphate,e.g., zinc phosphate or zinc-barium phosphate etc., zinc salts ofbenzoic acid or a substituted benzoic acid and an alkaline earth metalmetaborate. These three metal salts or pigments used alone and incombination with TiO₂ provide outstanding corrosion protection andenables the coating to be used as a self-priming, high-gloss orlow-gloss topcoat.

The preferred zinc salt of a substituted benzoic acid is specificallycharacterized as having at least one hydroxyl group and nitro (NO₂)substituent and molecular weights of about 100-500, e.g. 300, density ofabout 2-3 grams per mililiter and a specific surface area of 16M ² /gram. The benzoic acid salts are commercial products obtained from BASFand identified as Sicorin-RZ.

The preferred zinc phosphates, are complex alkaline-earth zincphosphates, e.g., zinc-barium phosphate. This particular zinc phosphateis available a Phos-Plus (J0866) from Mineral Pigments Corporation.

The alkaline metal metaborates, i.e., barium metaborate has thefollowing physical data:

    ______________________________________                                        PHYSICAL DATA                                                                 (BUTROL-22)                                                                   ______________________________________                                        Appearance:                                                                              White powder Bulk density: 3.65-3.75                               Odor:      None         pH (saturated solution):                                                      8.9-9.2                                               Fusion temp.:                                                                            1650-1920° F.                                               Solubility:                                                                              Less than 0.2% in water by weight                                  ______________________________________                                        FIRE AND EXPLOSION DATA                                                       ______________________________________                                        Flash point: Not applicable. Flammable limits: N/A                            Extinguishing Media: Not applicable.                                          Special Firefighting Procedures: None                                         ______________________________________                                        REACTIVITY DATA                                                               ______________________________________                                        Stability: Stable Incompatibility: None                                       Hazardous Decomposition Products: None                                        ______________________________________                                    

In addition to utilizing the above combination of metal pigments in therequired ratios, titanium dioxide is added to the coating to providereinforcing strength and to add color, hiding and opacity to thecoating. Other additives that maybe used include tinting or coloringagents which may be added to the coating in small but effective amountssuch as zinc oxide, antimony oxides, barium sulfate, calcium carbonateand one or more of the organic pigments such as the phthalocyaninecolors e.g. greens or blues, etc.

Specifically, the corrosion resistant coatings of this invention can beprepared by milling the ingredients set forth in the following Examples.

    ______________________________________                                                          EXAMPLE 1   EXAMPLE 2                                                         Parts by    Parts by                                                          Weight      Weight                                          Ingredients       (Ranges)    (Ranges)                                        ______________________________________                                        Polyurethane Resin                                                                              25-55       30-50                                           derived from                                                                  polyester polyols                                                             and polyisocyanates                                                           Titanium dioxide  5-40        10-35                                           Titanium dioxide   0-5.0        0-1.5                                         (vesiculated beads)                                                           Alkaline earth    1-30         5-25                                           metal phosphates and complexes                                                such as zinc and/or barium                                                    phosphates                                                                    Zinc salts of substituted                                                                       0.1-5       0.5-3                                           benzoic acids                                                                 Alkaline Earth    5-35        10-30                                           Metal Metaborates                                                             Dispersing agents,                                                                              0-3         0.1-1.0                                         e.g., Anti-Terra-204                                                          (Oil Soluble Anionic or                                                       Nonionic dispersants)                                                         Organic solvents for coatings                                                                   0-50        15-35                                           ______________________________________                                        Ingredients              Parts by Weight                                      ______________________________________                                        EXAMPLE 3                                                                     Polyurethane Resin derived from                                                                        38.7                                                 polyester polyol in organic solvents                                          (i.e., propylene glycol monomethylether acetate,                              N-butyl acetate) and polyisocyanate                                           Titanium Dioxide Pigment 10.1                                                 Titanium Dioxide Vesiculated Beads                                                                     0.6                                                  Zinc-Barium Phosphate    24.2                                                 Zinc Salt of a substituted Benzoic Acid                                                                2.4                                                  (Sicorin RZ)                                                                  Barium Metaborate        23.7                                                 (BUTROL-22)                                                                   Dispersant (BYK-320)     0.3                                                  Polyether Modified Methylalkyl Polysiloxane                                   Organic Solvents         As Needed                                            EXAMPLE 4                                                                     Polyurethane Resin derived from                                                                        49.40                                                79% solids solution polyester polyol in                                       organic solvents (propylene glycol monomethyl                                 ether acetate, n-butyl acetate) and                                           100% solids of a polyisocyanate                                               Titanium Dioxide Pigment 31.9                                                 Titanium Dioxide Vesiculated Beads                                                                     0.0                                                  Zinc-Barium Phosphate    6.2                                                  Zinc Salt of a substituted Benzoic Acid                                                                0.6                                                  (Sicorin RZ)                                                                  Barium Metaborate        11.5                                                 Dispersant (Anti-Terra-204)                                                                            0.3                                                  Organic Solvents         As Needed                                            EXAMPLE 5                                                                     Polyurethane Resin derived from                                                                        32.2                                                 79% solids solution of polyester polyol in                                    organic solvents (propylene glycol monomethyl                                 ether acetate, n-butyl acetate) and                                           100% solids of a polyisocyanate                                               Titanium Dioxide pigment 14.5                                                 Titanium Dioxide Vesiculated Beads                                                                     0.5                                                  Zinc-Barium Phosphate    22.4                                                 Zinc Salt of a substituted Benzoic Acid                                                                2.2                                                  (Sicorin RZ)                                                                  Barium Metaborate        28.0                                                 Dispersant               0.3                                                  Organic Solvents         As Needed                                            EXAMPLE 6                                                                     Polyurethane Resin derived from 79% solids                                                             41.2                                                 solution of                                                                   polyester polyol in organic solvents (propylene                               glycol monomethyl ether acetate, n-butyl                                      acetate) and 100% solids of a polyisocyanate                                  Titanium Dioxide         14.0                                                 Titanium Dioxide Vesiculated Beads                                                                     1.4                                                  Zinc-Barium Phosphate    23.3                                                 Zinc Salt of a substituted Benzoic Acid                                                                2.3                                                  (Sicorin RZ)                                                                  Barium Metaborate        17.6                                                 Dispersants              0.3                                                  (BYK-320 and Anti Terra-204)                                                  Organic Solvents         As Needed                                            ______________________________________                                    

In the examples, the polyester polyol blend was used as a solution,e.g., 79% solids in propylene glycol monomethyl ether acetate and butylacetate. The polyisocyanate was 100% solids, e.g., substantiallycontaining no solvents.

Preferably, the coatings are prepared by mixing all of the ingredients,except the polyisocyanate and then milling the mixture to a fineness ofabout 5 for camouflage and 7 for high gloss colors on the Hegman scaleaccording to ASTM D1210. Subsequently, the polyisocyanate is addedbefore the application of the coating to the substrate. The coating isapplied on the substrate at thickness ranging from about 0.001 to 0.003inches e.g. from 0.5 to about 10 mils and preferably 1 to 3 mils. Thecoating may be applied by various methods including spraying, rolling,or brushing onto the surface depending on the viscosity. The viscosityof the coating for the particular application may be achieved byadjusting the content of the solvent within the ranges specified hereinand by the selection of the particular reactants used to form theurethane resin. After the coating is applied to the surface, the solventis evaporated at room or elevated temperatures and is allowed to cure toa film thickness having the desired properties. The pigments can beintroduced into the coating by first forming a mill base with thepolyester polyol. The mill base can be formed, for example, byconventional sand-grinding or ball-milling techniques, and then blended,by simple stirring or agitation with the other ingredients of thecomposition.

The combination of the barium metaborate, the zinc salts of benzoicacid, e.g. zinc benzoate and an alkaline earth metal phosphate, e.g.,zinc or zinc-barium phosphates, improved the corrosion resistance whilemaintaining all the other desirable characteristics required of thecoating. In other words, the specific combination of the bariummetaborate, the zinc salt of substituted benzoic acids and zinc orzinc-barium phosphates, in the ratios stated, improved the corrosioninhibition substantially when compared to the use of either one of thesemetal salts alone in the same coating.

More specifically, in preparing the urethane resins, the preferredpolyester polyols of this invention have equivalent weights ranging fromabout 260 to 970 with hydroxyl numbers ranging from 40 to 252 and anacid number less than 10. The polyols includes a variety of polyesterpolyhydroxyl compounds known in the art including, for example, thecondensation-reaction products of pentaerythritol and/or glycols withmonocarboxylic acids or an aromatic or aliphatic dicarboxylic acid. Anybranched-chain glycol maybe used in the formation of the polyester,although it is preferred that these glycols contain no more than 8carbon atoms. A useful polyol is formed where the molar ratio of glycolto pentaerythritol is from 2:1 to about 6:1. The carboxylic acidcomponent of the polyester polyol prevents the molecular weight build-upof the polyol. It has been found that any aromatic or aliphaticmonocarboxylic acid or mixtures of these having 18 or less carbon atomscan be used. Normally, the acids are used in a molar ratio of acid topolyalcohol of about 1:1 to 2.5:1.

Examples of aromatic monocarboxylic acids include benzoic acid,butylbenzoic acid, triethyl benzoic acid, toluic acid, phenylaceticacid, and the like. Examples of aliphatic acids are acetic acid,propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid,pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid,stearic acid, etc.

The dicarboxylic acids useful in the formation of the polyester polyolshave the general formula:

    HOOC--R--COOH

where R is aliphatic or aromatic group. Preferred are succinic acid,glutaric acid, adipic acid and pimelic acid. Useful acids are those inwhich R has 2 to 8 carbon atoms with the preferred being maleic acid anditaconic acid. The aromatic dibasic acids are phthalic, isophthalic, andterephthalic, although other aromatic dibasic acids can be used.

It is known that the lower alkyl mono- or diesters of these acids andthe anhydrides thereof can be used in place of the free acids. Otherknown polyester polyols can be obtained by the condensation reactionbetween a polybasic acid, such as adipic acid, phthalic anhydride,isophthalic acid, etc., and a diol or triol, such as ethylene glycol,diethylene glycol, propylene glycol, trimethylol propane, glycerine,etc.

The hydroxyl numbers of the preferred polyester polyols should be atleast 40 and preferably between 40 and 252. The polyesters, containinghydroxyl groups, are combined with the polyisocyanate. This combinationcan be carried out in several ways known to the art. For example, anorganic solution of the polyester containing, if desired, acatalyst-promoting urethane formation such as an organo-tin compound, isadded to an chemical equivalent amount of the isocyanate. Thecombination is made at ambient temperature but the heat of reactionusually causes an increase in temperature. The mixture is agitatedpreferably at room temperature until the urethane reaction issubstantially completed. The course of the reaction can be followed bynoting the viscosity of the mixture. When the viscosity becomessubstantially constant, it may be concluded that the reaction issubstantially completed. The resultant reaction product may containinsignificant amounts of free isocyanate and/or hydroxyl groups.

Alternatively, the polyester solution can be reacted with a smallexcess, e.g. about 10% excess of the isocyanate. After the urethanereaction is substantially completed, the excess NCO groups can bereacted with "chain-extending" substances, e.g. water, alcohols, etc.This procedure results in polymers of substantially equivalent characterbut permits the reaction to proceed at a faster rate, due to the massaction of the excess NCO groups. The term "small excess" is intended tobe included within the meaning of the term "stoichiometric amounts".

The polyisocyanates and particularly the aliphatic polyisocyanates basedon HDI include various multi-functional aliphatic polyisocyanates havingan isocyanate content (NCO) ranging from about 10 to 30% by weight withan equivalent weight (NCO) ranging up to about 300. Specific examples ofthe organic polyisocyanates used in this invention make up about 5 to30% and preferably 5 to 20% by weight of the film-forming blend. Thesecompounds include aliphatic, cycloaliphatic, alkaryl, aralkyl,heterocyclic, and aryl di- or triisocyanates. Specific compoundsinclude, for example, polyisocyanates such as:

diphenylmethane-4,4'-diisocyanate,

diphenylene-4,4'-diisocyanate

toluene-2,4-diisocyanate,

toluene-2,6-diisocyanate,

3,3'-dimethoxy-4,4'-diphenylene diisocyanate methylene-bis-(4-cyclohexylisocyanate)

tetramethylene diisocyanate,

hexamethylene diisocyanate,

decamethylene diisocyanate,

ethylene diisocyanate,

ethylidene diisocyanate,

propylene-1,2-diisocyanate,

cyclohexylene-1,2-diisocyanate,

m-phenylene diisocyanate,

p-phenylene diisocyanate, 1,5-naphthalene diisocyanate,

3,3'-dimethyl-4,4'-biphenylene diisocyanate,

3,3'-dimethoxy-4-4'-biphenylene diisocyanate,

3,3'-diphenyl-4,4'-biphenylene diisocyanate,

4,4'-biphenylene diisocyanate,

3,3'-dichloro-4,4'-biphenylene diisocyanate,

furfurylidene diisocyanate,

bis-(2-isocyanatoethyl)fumarate,

1,3,5-benzene triisocyanate,

para, para', para"-triphenylmethane triisocyanate,

3,3'-diisocyanatodipropyl ether,

xylylene diisocyanate,

B,B-diphenyl propane-4,4'-diisocyanate, and

isophorone diisocyanate. Preferred polyisocyanates include hexamethylenediisocyanate and methylene-bis-(4-cyclohexyl isocyanate) e.g.DESMODUR-N.

By selecting the proper polyols and by adjusting the NCO to OH ratio,the physical properties and efficiency of the film, such as the strengthof film, flexibility, chemical resistance, solvent resistance, etc., canbe controlled over a wide range. Compounds where the NCO to OH ratioranges from 0.85 to 1.4 of NCO to 1.0 of OH groups e.g. 1.2:1 are usefulfor the manufacture of coating in accordance with this invention.

If the coating is derived from a two-package system, the polyisocyanateis in one package and a solution of the polyol is in a separate package.The two reactants, one containing the pigments, are thoroughly mixedjust before applying the coating onto the surface. Separation of the tworeactants is usually necessary since the "pot life" of some of thecompositions is short. The polyisocyanate (NCO) reacts with the hydroxylgroups of the polyol at temperature as low as about 40° F. (4° C.).Regardless of the method by which the coating composition is prepared,the coating should contain 20 to 60 parts by weight of the polyurethaneresin and up to about 50 parts, e.g. 0-50 parts by weight of solvent.The solvent of the composition can be a mixture of organic solventswherein the constituents of the urethane react.

Instead of the two-component or "two-package" system, a "one package"coating can be used if the reactive groups of the polyisocyanate areblocked with a blocking agent such a methylethyl ketoxime. Thiseliminates the need for keeping the polyol apart from the polyisocyanateuntil just before use. When the coating, with the blockedpolyisocyanate, is applied and heated the blocking agent is released,permitting the polyisocyanate to react with the polyester polyol.

The blocking agents are used for purposes of masking the free isocyanateradical of the polyisocyanates. These agents include phenol,m-nitrophenol, p-chlorophenol, ethyl malonate, acetylacetone, ethylacetoacetate, cresol, methanol, ethanol, ethylene, chlorophydrin, etc.Although the temperatures at which the above-mentioned blocking agentsare dissociated varies with the agents, it is generally accepted thatheating is required to deblock.

The coating composition also can contain ultraviolet light stabilizers,antioxidants, catalysts, wetting or dispersing agents, e.g.,Anti-Terra-204 (carboxylic acid salts of polyamine-amides), flowmodifiers e.g. BYK-320 (polyether modified methylalkyl polysiloxanecopolymers), adhesion promoters, etc. The ultraviolet light stabilizercan be present in an amount of 1-10% by weight, based on the weight ofthe urethane binder. The antioxidants can be present also in amounts of0.1-3% by weight of the urethane binder. Ultraviolet light stabilizersinclude benzophenones, triazoles, triazines, benzoates, substitutedbenzenes, organophosphorous sulfides, etc.

The coating composition of this invention may contain about 0.01-2.0% byweight, based on the weight of the polymer forming blend, of a curingcatalyst. The catalysts are usually organo metallics such as dibutyl tindilaurate and zinc octoate, dibutyl tin di-2-ethylhexoate, stannousoctoate, stannous oleate, zinc naphthenate, vanadium acetyl acetonate,and zirconium acetyl acetonate. Also useful as catalysts are tertiaryamines, such as, for example, triethylene diamine, triethylamine,pyridine, dimethylaniline, and methyl morpholine. When a two-componentsystem is used, the catalyst can be added to either the polyisocyanateor the solution of the polyester polyol.

The coating composition of this invention can be applied to a variety ofsubstrates by conventional application methods such a spraying, dipping,brushing, or flow coating. Substrates that can be coated with thecomposition are, for example, metal, wood, glass, or plastics such aspolypropylene, polystyrene, and the like. The coating is particularlysuited for application over pretreated or unprimed metal. The coatingcan be cured at ambient temperatures or heated at 40°-120° C. for up toan hour or more. If the coating contains a blocked polyisocyanate,temperatures ranging up to about 160° C. may be necessary.

The solvent may include a mixture of organic solvents, e.g., benzene,toluene, xylene, and naphtha. Ester solvents include the acetates, e.g.,ethyl acetate, butyl acetate, hexyl acetate, amyl acetate, etc.,propionates such as ethyl propionate, butyl propionate, etc. Ketonesolvents include acetone, methyl-ethyl ketone, methyl-isopropyl ketone,methylisobutyl ketone, diethyl ketone, cyclohexanone, etc. Glycol estersolvents include ethylene glycol, monoethyl-ether acetate, etc.

In testing the coatings prepared in accordance with this invention, thecorrosion protection for an aluminum substrate was found to be over 2000hours in 5% salt spray in accordance with ASTM Test Method B-117 andover 500 hours in SO₂ /salt spray in accordance with ASTM Test MethodG-85. The coating was found to have outstanding performance when exposedto extreme heat conditions, high intensity of light and water, extremecold conditions, hot lubricating oils and other chemicals normally foundin aircraft operations. By utilizing the coating composition of thisinvention, a corrosion resistant film can be obtained on varioussubstrates. The coating therefore has properties which function as aprimer and more important as a single topcoat which is highly adherent,flexible, chemical resistant and resistant to all weather conditions.The coatings of this invention lower the risk of failure due to crackingespecially at low temperatures and are easily touched-up since only onecoating need be applied. Since the coating requires only one coat, itrequires less time for application and removal and thereby saves onmanpower that would generally be needed in the preparation of a two coatsystem. Moreover, the present coating provides protection at lower filmthicknesses thereby reducing the weight of the coating compared to atwo-coat paint system which is an important factor when consideringaircraft coatings.

It is obvious that there are other variations and modifications whichcan be made with respect to this invention without departing from thespirit and scope of the invention as particularly set forth in theappendant claims.

The invention claimed:
 1. A corrosion-resistant self-priming coatingcomprising from about 25 to 55 parts by weight of a polyurethane resinbinder, 1 to 30 parts by weight of an alkaline earth metal phosphate,0.1 to 5 parts by weight of a zinc benzoate, 5 to 35 parts by weight ofan alkaline earth metal metaborate, 5.0 to 40 parts by weight oftitanium dioxide, 0 to 3.0 parts by weight of an oil soluble dispersingagent and 0 to 50 parts by weight of at least one organic solvent. 2.The coating of claim 1 wherein the polyurethane ranges from about 30 to50 parts by weight, titanium dioxide ranges from about 10 to 35 parts byweight, alkaline earth metal phosphate ranges from about 5 to 25 partsby weight, zinc benzoate ranges from about 0.5 to 3.0 parts by weight,alkaline earth metal metaborate ranges from about 10 to 30 parts byweight, the dispersing agent ranges from 0.1 to 1.0 parts by weight andthe solvent ranges from about 15 to 35 parts by weight.
 3. The coatingof claim 1 wherein up to about 100 percent by weight of the total amountof titanium dioxide is in the form of vesiculated beads.
 4. The coatingof claim 2 wherein 0 to 50 percent by weight of the total amount oftitanium dioxide is in the form of vesiculated beads.
 5. The coating ofclaim 4 wherein the zinc benzoate is a salt of a substituted benzoicacid having one hydroxyl substituent and one nitro substituent.
 6. Thecoating of claim 1 wherein the polyurethane is derived fromhexamethylene diisocyanate and an aliphatic polyester polyol.
 7. Thecoating of claim 6 wherein the polyurethane is derived from apolyisocyanate and a polyester polyol wherein the NCO to OH group ratiosrange from 0.85-1.4 to 1.0.
 8. The coating of the claim 6 wherein theNCO to OH ratio of the hexamethylene diisocyanate and the aliphaticpolyester polyol is about 1.2 to 1.0.
 9. The coating of claim 8 whereinthe zinc salt of the benzoic acid has one hydroxyl and one nitro (NO₂)substituent.
 10. The coating of claim 9 wherein the polyester polyol andthe polyisocyanate are reacted at a NCO to OH ratio of about 1.2 to 1.0.11. The coating of claim 2 wherein the alkaline earth metal phosphate isa zinc-barium phosphate.
 12. The coating of claim 2 wherein the alkalineearth metal phosphate is zinc phosphate.
 13. The coating of claim 1wherein the alkaline earth metal metaborate is barium metaborate.